Semiconductor optical device and method of manufacturing the same

ABSTRACT

A semiconductor optical device has a gain region for oscillating a laser light beam and a spot-size conversion region for converting a spot-size of the laser light beam emitted from the gain region. Further, an optical waveguide is formed by the use of a selective growth mask along the gain region and the spot-size conversion region. With such a structure, the optical waveguide includes a waveguide taper portion and has a width and a facet. In this event, the width gradually becomes narrower towards the facet. As a result, the waveguide taper portion is tapered along a direction from the gain region towards the facet.

BACKGROUND OF THE INVENTION

This invention relates to a semiconductor optical device which areapplicable for an optical communication, an optical disk device and anoptical interconnection and the like and a manufacturing method thereof,and in particular, to the semiconductor optical device which has aspot-size conversion function and a manufacturing method thereof.

In a semiconductor optical device, such as, a semiconductor laser, asemiconductor optical amplifier and a semiconductor optical modulator, aspot diameter of an optical beam which is emitted from an opticalwaveguide is small and further, a beam divergence is large.Consequently, it is generally difficult to couple the semiconductoroptical device to an optical fiber or a silica-based optical waveguide.

To this end, the semiconductor optical device is conventionally coupledto the optical fiber or the optical waveguide by the use of an opticalmodule with a lens. However, the lens if generally expensive, andfurther, the position of semiconductor optical device must be adjustedwith parts, such as the lens, the optical fiber and the opticalwaveguide at a high accuracy. This remarkably increases the price of theoptical module.

In this event, if the optical spot-size is enlarged at a facet of thesemiconductor optical device and further, the beam divergence becomesnarrow, the semiconductor optical semiconductor device can be coupled tothe optical fiber at the high efficiency without the positionaladjustment due to the high accuracy by the use of the expensive lens.Consequently, it may be possible to largely reduce the price of theoptical module.

From the above-mentioned reasons, various suggestions has beenconventionally made about the semiconductor optical device having thespot-size conversion function.

For instance, suggestion has been made about a semiconductor opticaldevice of the Fabry-perot laser (thereinafter, referred to as FP-LD, andcalled a first conventional reference) in Japanese UnexaminedPublication No. Hei. 7-283490. In this FP-LD, the waveguide of thesemiconductor is integrated to convert the optical spot-size. The FP-LDhas a gain region and a spot-size conversion region on a semiconductorsubstrate. With such a structure, the spot-size is enlarged by changingthe layer thickness of the optical waveguide to realize a narrow beamdivergence in the above spot-size conversion region.

On the other hand, another suggestion has been made about another FP-LD(thereinafter, called a second conventional reference) in ElectronicsLetters August 1996, Vol. 31 No. 17, pp. 1439-1440. In FP-LD of thesecond conventional reference, the optical spot-size is enlarged at thelaser output facet by the use of a lateral direction taper shape. Inthis event, the lateral direction taper shape is formed by etching anepitaxial layer which is flatly grown on the entire surface of thesubstrate without using the selective growth method.

In the first conventional reference, the spot-size conversion region(namely, an active region) must be formed within the length between 200μm and 300 μm. Consequently, the device yield for each wafer is reduced.Further, the photo-lithography steps must be twice carried out to formthe selective growth mask, and the mesa-etching process must be alsoperformed to form the waveguide. As a result, the manufacturing processinevitably becomes complicated.

On the other hand, the above-mentioned problem may be solved because thelateral taper shape having the optical gain is formed by etching thesemiconductor active layer in the second conventional reference.However, the semiconductor layer must be processed in a sub-micron orderat the tip portion of the tapered waveguide. Consequently, it isdifficult to form the waveguide at the high accuracy by the use of thedry method in addition to the wet method. As a result, it is alsodifficult to uniformly fabricate the taper shape and to excellentlyreproduce the device characteristic.

Moreover, the device characteristic including the beam divergencelargely depends upon the stripe width of the active layer in the firstconventional reference. Consequently, it is difficult to stablyfabricate the device having the narrow beam divergence on the conditionthat the excellent characteristic, the reproducibility and theuniformity of the shape are kept.

Further, the process accuracy is slightly increased in the dry method ascompared to the patterning due to the wet method to form the opticalwaveguide. However, the active layer is damaged from the side surface inthis case. Moreover, it is difficult to excellently form a buried layerat the side surface of the optical waveguide layer during growing theburied layer which is carried out after patterning the optical waveguidelayer. Consequently, it is also difficult fabricate the device at a highreliability.

To avoid this problem, the wet process must be carried out to remove thedamaged layer after the dry-etching process. Finally, the high processaccuracy can be practically obtained.

Further, when the coupling with the optical fiber is taken into account,it is desirable that the optical spot is formed into an approximatelycircular shape at the output facet with the small emission angle.However, the circular spot shape is realized only by changing the layerthickness like the first conventional reference or by forming thelateral taper shape like the second conventional reference.Consequently, the design flexibility of the device parameters, such as,the active layer structure, the active layer width and the taper shape,is remarkably restricted.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a semiconductoroptical device which has a spot-size conversion function and which iscapable of emitting an optical beam having an approximately circularspot shape with a narrow beam divergence.

It is another object of this invention to provide a method which iscapable of manufacturing a semiconductor optical device having aspot-size conversion function with excellent reproducibility anduniformity of a taper shape and a device characteristic.

According to this invention, a semiconductor optical device has a gainregion for oscillating a laser light beam and a spot-size conversionregion for converting a spot-size of the laser light beam emitted fromthe gain region. Further, an optical waveguide is formed by the use of aselective growth mask along the gain region and the spot-size conversionregion.

With such a structure, the optical waveguide includes a waveguide taperportion and has a width and a facet. In this event, the width graduallybecomes narrower towards the facet. As a result, the waveguide taperportion is tapered along a direction from the gain region towards thefacet.

More specifically, both the mask width and the opening width of theselective growth mask are formed in the taper form. Further, the opticalwaveguide is directly formed only by the use of the selective growthwithout the etching process. If the optical waveguide is formed by theuse of the etching process, the epitaxial growth layer having the layerthickness of between about 0.6 μm and 1.5 μm must be etched. It isdifficult to process the semiconductor layer having the above layerthickness in the sub-micron order with the accuracy and the excellentreproducibility.

On the other hand, when the optical waveguide is formed by the use ofthe selective growth, the accuracy, the reproducibility and theuniformity of the optical waveguide depends upon the accuracy, thereproducibility and the uniformity of the selective growth mask.

consequently, the semiconductor layer can be processed in the sub-micronorder at the high accuracy and the reproducibility. This is also becausethe selective growth mask is formed by a dielectric thin layer havingthe layer thickness between 0.05 μm and 0.1 μm, such as, a SiO₂ film anda Si₃N₄ film, and further, the etching depth becomes about {fraction(1/10)} as compared to the case of the etching process of the epitaxiallayer. Namely, the dimension accuracy and the dimension variation can bealso set to about {fraction (1/10)} as compared to the conventionalcase.

Moreover, the stripe width of the optical waveguide which is formed bythe selective growth becomes narrower than the opening width of theselective growth mask. Consequently, the stripe can be formed with thedimension which is less than the resolution limit determined in thephoto-lithography, for example, the dimension which is less than thequarter micron, with the high accuracy and the excellentreproducibility.

Further, the mask width itself in addition to the mask opening widthgradually becomes narrower towards the facet direction in the selectivegrowth mask which determines the lateral width dimension of thesemiconductor waveguide. By adopting this method, the design flexibilityis remarkably enhanced. Further, the optical spot-size can beefficiently enlarged in accordance with the waveguide length of theshort spot-size converter. Moreover, the design can be easily carriedout to obtain the circular optical spot shape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross sectional perspective view showing a semiconductoroptical device according to a first conventional reference;

FIG. 1B is a plan view for explaining a method of manufacturing asemiconductor optical device according to a first conventionalreference;

FIG. 2 is a perspective view showing a semiconductor optical deviceaccording to a second conventional reference;

FIG. 3 is a perspective view showing a semiconductor optical deviceaccording to a first embodiment;

FIG. 4A is a perspective view showing a semiconductor optical deviceaccording to a second embodiment;

FIG. 4B is a plan view showing a semiconductor optical device accordingto a second embodiment;

FIG. 4C is another plan view showing a semiconductor optical deviceaccording to a second embodiment;

FIG. 5A is a perspective view showing a semiconductor optical deviceaccording to a third embodiment;

FIG. 5B is a plan view showing a semiconductor optical device accordingto a third embodiment;

FIG. 6A is a plan view for explaining a step of manufacturing asemiconductor optical device according to a first example correspondingto the first embodiment illustrated in FIG. 3;

FIGS. 6B through 6D are cross sectional views for explaining steps ofmanufacturing a semiconductor optical device according to a firstexample;

FIGS. 7A through 7C show a L-I (light output power-injection current)characteristic and an optical strength distribution of a far-fieldpattern of a semiconductor laser having a narrow beam divergenceaccording to a first example;

FIG. 8A is a profile showing a layer thickness in a resonator directionand a band gap wavelength (photo-luminescence wavelength) according to afirst example;

FIG. 8B is a plan view showing a semiconductor optical device accordingto a first example;

FIG. 9A shows an example of a mask shape for manufacturing a lateraltaper type semiconductor layer having a narrow beam divergence accordingto a first example;

FIG. 9B shows another example of a mask shape for manufacturing alateral taper type semiconductor laser having a narrow beam divergenceaccording to a first example;

FIG. 10A is a plan view for explaining a step of manufacturing asemiconductor optical device according to a second example correspondingto the first embodiment illustrated in FIG. 3;

FIGS. 10B through 10D are cross sectional views for explaining steps ofmanufacturing a semiconductor optical device according to a secondexample;

FIG. 11 is a profile showing a layer thickness in a resonator directionand a band gap wavelength (photo-luminescence wavelength) according to asecond example;

FIGS. 12A through 12C show a L-I characteristic and an optical strengthdistribution of a far-field patter of a semiconductor laser having anarrow beam divergence according to a second example;

FIG. 13A is a plan view for explaining a step of manufacturing asemiconductor optical device according to a third example correspondingto the second embodiment illustrated in FIG. 4;

FIGS. 13B through 13D are cross sectional views for explaining steps ofmanufacturing a semiconductor optical device according to a thirdexample;

FIG. 14A is a plan view for explaining a step of manufacturing asemiconductor optical device according to a fourth example correspondingto the third embodiment illustrated in FIG. 5; and

FIGS. 14B through 14D are cross sectional views for explaining steps ofmanufacturing a semiconductor optical device according to a fourthexample.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2, conventional semiconductor optical deviceswill be first described for a better understanding of this invention.The semiconductor optical devices are equivalent to the conventionalsemiconductor optical devices mentioned in the preamble of the instantspecification.

The FP-LD of the first conventional reference (disclosed in JapaneseUnexamined Publication No. Hei. 7-283490) will be manufactured asfollows.

In this FP-LD, the optical waveguide of the semiconductor is integratedto convert the optical spot-size. The FP-LD has a gain region 22 and aspot-size conversion region 23 on a semiconductor substrate 1, asillustrated in FIG. 1A.

As shown in FIG. 1B, a selective growth mask 2 is formed on thesemiconductor substrate 1 so that a constant opening width is kept inthe gain region 22 while the opening width is gradually widen towardsone facet in the spot-size conversion region 23.

Further, a clad layer 7, a separate confinement hetero-structure(thereinafter, abbreviated a SCH) layer 8, a multi-quantum well(thereinafter, abbreviated a MQW) layer 9, a SCH layer 8 and clad layerare successively deposited on the semiconductor substrate 1, asillustrated in FIG. 1A. In this event, an epitaxial layer is formed suchthat the layer thickness is gradually reduced towards the facet of thespot-size conversion region 23.

Subsequently, a stripe-shaped mask having the width of 1.5 μm is formedon the epitaxial layer which is selectively grown. Thereby, theepitaxial layer is patterned, and a current blocking layer 11 isselectively grown on the condition that the stripe shaped mask is left.

Successively, a contact layer 13 is formed thereon. Further, aninsulating layer 15 having an opening 21 only on the gain region 22 isformed thereon. Thereafter, electrodes 14 are formed on upper and lowersurfaces of the substrate 1.

With such a structure, the spot-size is enlarged by changing the layerthickness of the optical waveguide to realize a narrow beam divergencein the spot-size conversion region 23 in the first conventionalreference.

On the other hand, the FP-LD of the second conventional reference(disclosed in Electronics Letters August 1996, Vol. 31 No. 17, pp.1439-1440) will be manufactured as follows by using the manufacturingtechnique of the conventional buried hetero (thereinafter, abbreviatedas BH) structure LD, referring to FIG. 2.

An MQW layer is formed by the use of the epitaxial growth on asemiconductor substrate. In this event, the MQW layer is interposedbetween SCH layers. The epitaxial layer is patterned by the use of thephoto-lithography method and the etching method to form a semiconductoractive layer 24, as illustrated in FIG. 2. Herein, the semiconductoractive layer 24 is formed into a mesa shape and has a taper portion 5and a rear straight portion. Thereafter, a current blocking layer (notshown) is grown around the mesa stripe. Further, an over clad layer anda contact layer (not shown) are successively grown. Thus, the LDillustrated in FIG. 2 having a large spot-size is completed.

With such a structure, the optical spot-size is enlarged at the laseroutput facet by the use of the lateral direction taper shape in thesecond conventional reference. In this event, the lateral directiontaper shape is formed by etching the epitaxial layer which is flatlygrown on an entire surface of the substrate without using the selectivegrowth method.

If the coupling with the optical fiber is taken into account, it isdesirable that the optical spot is formed approximately circular shapeat the output end with the small emission angle. However, theapproximate circular spot is realized only by changing the layerthickness like the first conventional reference or by forming thelateral taper shape like the second conventional reference.Consequently, the design flexibility of the device parameters, such as,the active layer structure, the active layer width and the taper shape,is remarkably restricted.

Taking the above-mentioned problem into consideration, this inventionprovides a semiconductor optical device which has a spot-size conversionfunction and which is capable of emitting a optical beam having anapproximately circular spot-shape with a narrow beam divergence.

(First embodiment)

Referring to FIG. 3, description will be made about a semiconductoroptical device according to a first embodiment of this invention.

As illustrated in FIG. 3, a selective growth mask 2 having a tapershaped portion is formed so that both a mask opening width and a maskwidth are gradually reduced towards a forward facet of a semiconductorlaser. A semiconductor layer which constitutes an optical waveguide isformed by the use of the epitaxial growth with the above mask. Thereby,a mesa-type semiconductor optical waveguide 3 is directly formed on asemiconductor substrate 1. In this event, the mesa-type semiconductoroptical waveguide 3 has a portion 30 which is formed in a taper shape ina lateral direction or in vertical and lateral directions as a spot-sizeconversion region.

Namely, the optical waveguide having the spot-size conversion functionis formed in a lateral taper shape which has an uniform layer thicknessand a band gap wavelength in an optical waveguide direction in thesemiconductor optical device according to this invention. In this case,the taper portion 30 can be used as an active waveguide.

On the other hand, the optical waveguide may be formed in a vertical andlateral taper shape in which the waveguide width becomes narrower andthe layer thickness also becomes smaller to shorten a band gapwavelength towards the facet. In this event, the taper portion 30 can beused as a passive waveguide.

Herein, a multi-semiconductor layer of the optical waveguide isselectively and effectively grown by the use of the metal organic vaporphase epitaxy (thereinafter, abbreviated as a MOVPE). Alternatively, themolecular beam epitaxy (thereinafter, abbreviated as a MBE) may be used.In this case, the optical waveguide is formed by a quantum wellstructure or a multi-quantum well structure as a core layer.

More specifically, the quantum well structure is formed by arranging aquantum well structure, a multi-quantum well structure or an opticalconfined layer at an upper or a lower sides and both sides. Thisformation is similar to a second and third embodiments which will belater described.

Although the above opening width of the selective grown mask 2 can beoptionally designed, it is desirable that the opening width is set inthe range between 0 μm and 1 μm at a forward facet of the semiconductorlaser and in the range between 1 μm and 3 μm at a rear facet thereof.

The changing quantity of the mask width is set to a proper value betweenseveral μm and several tens μm for the change of the mask opening widthof about several μm. Thereby, the layer thickness and the band gapwavelength of the semiconductor active layer are kept constant in theresonator direction. Consequently, the lateral taper semiconductor laserhaving the narrow beam divergence can be fabricated. In this event, theactive layer having the optical gain serves as the spot-size conversionwaveguide.

On the other hand, the changing quantity of the mask width may bedesigned larger than the above value, for example, to several tens μm.Thereby, the semiconductor laser active layer having the optical gaincan be entirely grown to integrate with the spot-size conversion passivewaveguide of the lateral and vertical taper type. In this event, theoptical waveguide width, the layer thickness and the bank gap wavelengthbecome smaller towards the forward facet of the semiconductor laser.

In the first embodiment, the optical waveguide layer may be grown by theuse of the selective growth mask having no mask opening at the outputside facet like the subsequent second embodiment. Thereby, a windowstructure may be formed at the optical output facet portion without theetching process.

(Second embodiment)

Referring to FIGS. 4A through 4C, description will be made about asemiconductor optical device according to a second embodiment of thisinvention.

Both facets must be coupled to an optical fiber or a quarts-basedoptical waveguide in a semiconductor optical amplifier. In this event,an optical light beam is entered to one facet while the optical lightbeam which is amplified in an active layer is emitted from anotherfacet.

Consequently, the optical spot-sizes must be enlarged at the both facetsto produce an optical module with a low price.

As shown in FIG. 4B, the mask width and the mask opening width are keptconstant in a linear gain waveguide portion 17 and a facet straightportion 16 on a substrate 1. On the other hand, a selective growth mask2 is formed on the semiconductor substrate 1 so that both the mask widthand the mask opening width become narrower towards the facet in a taperportion 5. Further, the semiconductor layer is formed by the use of theepitaxial growth by using the selective growth mask 2 as a mask.Thereby, a mesa-type semiconductor optical waveguide 2 which has theoptical spot-size conversion (enlarging) function at the both facets isobtained. Specifically, taper shapes are formed in a lateral directionor vertical and lateral directions at the both surface sides in theabove mesa-type semiconductor optical waveguide 3.

The spot-size conversion region which is placed in the taper portion 5is formed as the optical waveguide which has the taper in only thelateral direction or the lateral and vertical directions. Morespecifically, the spot-size conversion region is formed in the lateraltaper type which has the approximately uniform layer thickness and theband gap wavelength composition. Alternatively, the spot-size conversionregion may be formed in the vertical and lateral taper type in which thelayer thickness becomes thinner and the band gap wavelength becomesshorter as the wavelength width becomes narrower towards the facet. Inthis event, the former can be used an active waveguide while the lattercan be used as a passive waveguide.

Alternatively, the optical waveguide having a window structure may beformed by using a selection mask which has no opening in a windowstructure portion 18, as illustrated in FIG. 4C. In this event, afterthe mesa-type semiconductor optical waveguide 3 is formed by the use ofthe epitaxial growth process, the selective growth mask 2 is removed.Thereafter, another selective growth mask (not shown) is formed on theoptical waveguide 3, and a buried layer is formed to cover a sidesurface and both facets of the optical waveguide layer.

Subsequently, the optical waveguide 2 having the window structureportion 18 is formed by cleaving the embedding layer in the forwarddirection of the waveguide facet.

Moreover, an electrical field absorb type modulator may be formedinstead of the optical amplifier in the second embodiment.

(Third embodiment)

Referring to FIGS. 5A and 5B, description will be made about asemiconductor optical device according to a third embodiment of thisinvention.

An optical modulator integration type semiconductor laser is efficientlycoupled to an optical fiber according to this invention. In this event,an optical output from a laser portion can be reduced. Herein, the laserportion must emit a laser light beam of a desired strength to theoptical fiber. As a result, a carrier pileup is suppressed at amodulator portion to reduce the drop of an extinction ratio caused bythe carrier pileup.

As shown in FIG. 5B, a selective growth mask 2 is formed on a substrate1. In this event, the mask opening width Wo of the selective growth mask2 is kept constant in a distributed feedback (thereinafter, abbreviatedas a DFB) laser portion 20 and an electrical absorb type modulatorportion 19, as illustrated in FIG. 5B.

Further, the mask opening width Wo gradually becomes narrower towards anoptical output facet in a taper portion 5. Moreover, the mask openingwidth Wo is kept constant at a forward straight portion 6.

On the other hand, the mask width Wm is widely formed in the DFB laserportion 20 while the mask width Wm is narrowly formed in the electricalabsorb type modulator portion 19. Thus, there is a step differencebetween the DFB laser portion 20 and the electrical absorb typemodulator portion 19. Further, the mask width Wm gradually becomesnarrower towards the optical output facet in the taper portion 5 whilethe mask width Wm is kept constant in the forward straight portion 6.

The semiconductor layer constituting the optical waveguide 3 is formedby the use of the epitaxial growth method on the substrate 1 by usingthe selective growth mask 2 as a mask. In this event, the layerthickness of the modulator portion 19 is thinner than that of the DFBlaser portion 20 to shorten the band gap wavelength, as shown in FIG.5A.

Further, the lateral taper type which has the approximately uniformlayer thickness and the band gap wavelength composition towards theoptical waveguide direction is formed in the taper portion 5. Herein,the optical waveguide has the spot-size conversion function in the taperportion 5.

Alternatively, the vertical and lateral taper type may be formed in thetaper portion 5. Herein, the waveguide width gradually becomes narrowertowards the facet and the layer thickness becomes thinner to shorten theband gap wavelength in the taper portion 5.

In this embodiment, the optical waveguide layer may formed by using theselective growth mask having no mask opening at the output facet.Thereby, the window structure may be formed at the optical output facetportion without the etching process like in the second embodiment.

First Example

Referring to FIGS. 6A through 6D, description will be made about asemiconductor optical device according to a first example of thisinvention. Herein, the first example corresponds to the first embodimentillustrated in FIG. 3.

As shown in FIG. 6A, a SiO₂ film having the layer thickness of 0.1 μm isdeposited by the use of the CVD method on an n-InP substrate 1 a and ispatterned by the use of the photo-lithography method and the dry-etchingmethod to form a SiO₂ mask 2 a. The mask width Wm and the opening widthWo of the mask 2 a are set to 10.0 μm and 2.0 μm in the rear straightportion 4 having a length of 75 μm, respectively. Further, the maskwidth Wm and the opening width Wo gradually become smaller, and are setto 5.0 μm and 0.7 μm in the forward straight portion 6 having the lengthof 25 μm, respectively.

In this case, the positioning accuracy of the laser facet which isnormally cleaved is equal to about several μm at best in the forwardstraight portion 6. To this end, this forward straight portion 6 isarranged to prevent an error for the active layer width. In this event,this error occurs in the forward output end in accordance with thecleavage positioning accuracy.

The error from the design value of the mask opening width Wo is equal to0.025 μm when the taper design of the opening portion is the same withthis invention and the facet position is deviated at 5 μm without theforward straight portion. Although the above positioning accuracy isslightly larger than the optical width accuracy of this example which isdetermined by the patterning accuracy of the selective growth mask 2 a,the forward straight portion 6 can be formed with a sufficiently highaccuracy as compared to the conventional case in which the opticalwaveguide width is determined by the etching quantity for thesemiconductor layer.

The length of the forward straight portion 6 is not limited to 25 μm inthis example, and may be set to 0 μm. Namely, the design can beoptionally carried out to include the case having no forward straightportion. Further, the presence or absence of the straight portion 6 and4 at the forward facet, the backward facet and the both surfaces canalso be optionally selected.

On the substrate 1 a having such a SiO₂ mask 2 a, an n-InP clad layer 7a (the thickness of 0.2 μm, the doping concentration of 1×10¹⁸ cm⁻³), ann-type InGaAsP-SCH layer 8 a (a PL (photo-luminescence) wavelength of1.05 μm composition, the thickness of 50 nm+the PL wavelength of 1.13 μmcomposition, the thickness of 10 nm), an InGaAs quantum well structurelayer (the thickness of 5 nm) with +0.7% compressive strain, a MQW layer9 (the number of quantum well of six, the PL wavelength of about 1.3 μm)consisting of an InGaAs P barrier layer (the PL wavelength of 1.13 μmcomposition, the thickness of 8 nm), a p-type InGa AsP-SCH layer 8 b(the PL wavelength of 1.13 μm composition, the thickness of 10 nm+the PLwavelength of 1.05 μm composition, the thickness of 50 nm), and a p-InPclad layer 7 b (the thickness of 0.1 μm, the doping concentration of7×10¹⁷ cm⁻³) are successively deposited under the composition and thelayer thickness design value in the rear straight portion 4 (Wm=10.0 μm,Wo=2.0 μm), as illustrated in FIG. 6B.

Herein, the center of the active layer consisting of the SCH layers 8 a,8 b and the MQW layer 9 is positioned at the height of 0.2 μm+{50 nm+10nm+(5 nm×6+8 nm×5)+10 nm+50 nm}/2=0.295 μm from the InP substrate 1 a.The mesa which is formed by the selective growth method is structured sothat the slope has an angle of 54.7° for the substrate 1 a.Consequently, the width of the active layer at this position becomesnarrower at 2×0.295 μm×cost 54.7°=0.417 μm than the opening width of theSiO₂ mask 2 a.

Therefore, the design value of the active layer width is equal to 2.0μm−0.417 μm=1.583 μm at the rear facet and 0.7 μm−0.417 μm=0.283 μm atthe forward facet in this example, when it is assumed that the selectivegrowth rate is kept constant independently of the position of theresonator.

Subsequently, after the SiO₂ mask 2 a is removed, another SiO₂ mask 10is formed only on the mesa type optical waveguide including the activelayer by the use of the CVD method and the photo-lithography, asillustrated in FIG. 6C. Successively, a current squeeze structure whichis composed of a p-InP blocking layer 11 a (the thickness of 0.5 μm, thedoping concentration of 3×10¹⁷ cm⁻³) and an n-InP blocking layer 11 b(the thickness of 0.7 μm, the doping concentration of 1×10¹⁸ cm⁻³) isdeposited on the substrate 1 a, as shown in FIG. 6C.

Next, a p-InP buried layer 12 and a p-InGaAS contact layer 13i a aresuccessively deposited after the SiO₂ mask 10 is removed, as shown inFIG. 6D. Thereafter, electrodes 14 are formed at upper and lowersurfaces of the substrate 1 a. Thus, the semiconductor laser having thenarrow beam divergence is completed, as illustrated in FIG. 6D.

In FIGS. 7A through 7C, a L-I characteristic and a beam divergencecharacteristic of the semiconductor optical device are exemplified.Herein, the semiconductor optical device is coated with a multi-filmwhich has a reflection rate of 95% at the rear facet after cleaving aresonator length to 250 μm. In this event, a threshold current is equalto 6.0 mA at 25° C., a slope efficiency is equal to 0.56 W/A, full widthat half maximum of the far-field pattern optical strength distributionis equal to 13.2° in the parallel direction and 15.4° in the verticaldirection with respect to the narrow beam divergence characteristic.

Further, the width of each of the active layers of thirty devices whichare taken out from arbitrary positions within radius 18 mm of threedifferent wafer has an average value of 0.288 μm and a standarddeviation of 0.018 μm at the forward facet, and the average value of1.587 μm and the standard deviation of 0.022 μm at the rear facet,respectively. Thus, the semiconductor optical device is excessivelysuperior in the uniformity and the reproducibility.

Each of three hundred devices which are taken out in the same manner hasthe standard deviation of 0.52 mA of the threshold current at 25° C.,the standard deviation of the slope efficiency of 0.041 W/A, thestandard deviation of full width at half maximum of the far-fieldpattern optical strength distribution of 0.5° in the parallel directionand 0.6° in the vertical direction. Thus, the semiconductor opticaldevice has excessively excellent uniformity and the reproducibility.

The layer thickness and the band gap wavelength of the active layerconsisting of the SCH layers 8 a and 8 b and the MQW layer 9 accordingto this example is kept constant in the resonator direction, asindicated by a solid line in FIG. 8A. In contrast, when only the openingportion having the taper shape in FIG. 6A is arranged on the rectangularSiO₂ mask 2 a as shown in FIG. 8B, the spot-size conversion efficiencyis remarkably reduced as indicated by a dot line in FIG. 8A. This isbecause the thickness of the active layer which is selectively grown isincreased as the opening portion becomes narrower. Further, the opticalgain wavelength of the entire resonator is dispersed because the bandgap wavelength becomes long. Consequently, the L-I characteristic of thesemiconductor laser is largely degraded.

The selective growth mask 2 a of this example is designed to avoid thedeterioration of the performance as the semiconductor laser having thenarrow beam divergence. Consequently, the active layer thickness and theband gap wavelength are kept almost constant. However, an optimum valueof the mask width Wm which is accompanied with the change of the openingwidth Wo is variable in accordance with the growth method and the growthcondition of the semiconductor optical waveguide including the activelayer in addition to the changing quantity of the opening width and thereferential mask width. Therefore, the dimension of the selective growthmask 2 a is not limited to the above-mentioned value, and should besuitably determined by taking the characteristic of the opticalwaveguide, the growth method and the growth condition of thesemiconductor layer into consideration.

Moreover, it is unnecessary that the layer thickness and the band gapcomposition become completely uniform in the optical waveguidedirection, and it is possible that the band gap composition has acertain degree of distribution.

Further, the taper shape is not limited to the linear shape asillustrated in FIG. 6A, and the taper shape may have a gently taper orthe different taper length towards the forward facet, as shown in FIGS.9A and 9B. At any rate, any shapes can be adopted as long as the opticalwaveguide includes the portion in which the mask width Wm and theopening width Wo gradually becomes smaller towards the facet.

(Second Example)

Referring to FIGS. 10A through 10D, description will be made about asemiconductor optical device according to a second example of thisinvention. Herein, the second example also corresponds to the firstemobdiment illustrated in FIG. 3.

As shown in FIG. 10A, a SiO₂ film having the layer thickness of 0.05 μmis deposited by the use of the CVD method on an n-InP substrate 1 a andis patterned by the use of the photo-lithography method and thedry-etching method to form a SiO₂ mask 2 a. The mask width Wm and theopening width Wo of the mask 2 a are set to 50.0 μm and 1.6 μm in therear straight portion 4 having the length of 190 μm, respectively.Further, the mask width Wm and the opening width Wo gradually becomesmaller in the taper portion 5 having the length of 150 μm, and are setto 6.0 μm and 0.6 μm in the forward straight portion 6 having the lengthof 10 μm, respectively.

On the substrate 1 a having such a SiO₂ mask 2 a, an n-InP clad layer 7a (the thickness of 0.2 μm, the doping concentration of 1×10¹⁸ cm⁻³), ann-type InGaAsP-SCH layer 8 a (the PL wavelength of 1.13 μm composition,the thickness of 50 nm), a MQW layer 9 (the number of quantum well ofeven, the PL wavelength of about 1.3 μm) consisting of an InGaAsPquantum well structure layer (the thickness of 5 nm) with +1.0%compressive strain and an InGaAsP barrier layer (the PL wavelength of1.1 μm composition, the thickness of 8 nm), a p-type InGa AsP-SCH layer8 b (the PL wavelength of 1.13 μm composition, the thickness of 50 nm)and a p-InP clad layer 7 b (the thickness of 0.1 μm, the dopingconcentration of 7×10¹⁷ cm⁻³) are successively deposited under thecomposition and the layer thickness design value in the rear straightportion 4 (Wm=10.0 μm, Wo=2.0 μm), as illustrated in FIG. 10B.

In this event, the layer thickness and the band gap wavelength of theactive layer having the SCH layers 8 a, 8 b and the MQW layer 9 areillustrated in FIG. 11. As shown in FIG. 11, the layer thickness of theoptical waveguide is reduced in the taper portion 5 of the mask 2 a andthe band gap wavelength is shortened. Thus, the excellent spot-sizeconversion passive waveguide having a small optical absorb loss can beobtained in this example.

Subsequently, a SiO₂ mask 10 having the opening width 1 μm forselectively growing a buried layer is formed on an n-InP substrate 1 aby the use of the CVD method, the photo-lithography and the dry-etchingmethod, as illustrated in FIG. 10C. Thereafter, a p-InP buried layer 12and a p-InGaAs contact layer 13 a are successively grown. Subsequently,an insulating film 15 is deposited, as illustrated in FIG. 10D. After anopening is formed to expose a surface a p-InGsAs contact layer 13 a,electrodes 14 are formed the upper and lower surfaces of the substrate 1a. Thus, the semiconductor laser of the vertical and lateral taper typehaving the narrow beam divergence is completed in this example, asillustrated in FIG. 10D.

In this event, the length of the opening which is formed in theinsulating film 15 is equal to 220 μm at the total of 190 μm at the rearstraight portion 4 and 30 μm at the rear side of the taper portion 5.Thereby, the deterioration of the L-I characteristic which is caused bya supersaturation absorb phenomenon is avoided at the taper rear portionhaving the band gap wavelength relatively close to the oscillationwavelength of the laser.

In FIGS. 12A through 12C, a L-I characteristic and a beam divergencecharacteristic of the semiconductor optical device rear indicated.Herein, the semiconductor optical device is coated with a multi-filmwhich has a reflection rate of 95% at a rear facet after cleaving aresonator length to 360 μm. In this event, the threshold current isequal to 8.0 mA at 25° C., the slope efficiency is equal to 0.45 W/A,the full width at half maximum of the far-field pattern image opticalstrength distribution is equal to 10.8° in the parallel direction and10.6° in the vertical direction. Thus, the narrow beam divergencecharacteristic has an excellent aspect ratio.

Further, the width of each of the active layers of thirty devices whichare taken out from arbitrary positions within radius 18 mm of threedifferent wafer has an average value of 0.224 μm and a standarddeviation of 0.021 μm at the forward facet, and the average value of1.177 μm and the standard deviation of 0.024 μm at the rear facet,respectively. Thus, the semiconductor optical device is excessivelysuperior in the uniformity and the reproducibility.

Each of three hundred devices which are taken out in the same manner hasthe standard deviation of 0.62 mA of the threshold current at 25° C.,the standard deviation of the slope efficiency of 0.041 W/A, thestandard deviation of the full width at half maximum of the far-fieldpattern optical strength distribution of 0.4° in the parallel andvertical directions. Thus, the semiconductor optical device hasexcessively excellent uniformity and the reproducibility.

In this event, the full width at half maximum of the far-field patternoptical strength distribution of the spot-size conversion waveguideintegration laser which has only the vertical direction taper and whichis manufactured on the condition that the opening width Wo is constantlyset to 1.6 μm at the design of the same mask width with this example isequal to 13° in the parallel and 15° in the vertical direction. As aresult, it is confirmed that the spot-size conversion waveguideintegration laser having the vertical and lateral taper type isadvantageous in the spot-size conversion efficiency and the aspect ratioof the optical spot.

(Third Example)

Referring to FIGS. 13A through 3D, description will be made about asemiconductor optical device according to a third example of thisinvention. Herein, the third example corresponds to the secondemobdiment illustrated in FIG. 4.

As shown in FIG. 13A, a SiO₂ film having the layer thickness of 0.1 μmis deposited by the use of the on an n-InP substrate 1 a and ispatterned by the use of the photo-lithography method and the dry-etchingmethod to form a SiO₂ mask 2 a. The mask width Wm and the opening widthWo of the mask 2 a are set to 50.0 μm and 2.0 μm in the lenear gainwaveguide portion 17 having the length of 190 μm, respectively. Further,the mask width Wm and the opening width Wo gradually become smaller inthe taper portions 5 having the length of 130 μm, and are set to 1.0 μmand 0.6 μm in the portions which contact with the window structureportions 18 having the length of 10 μm, respectively.

On the n-InP substrate 1 a having such a SiO₂ mask 2 a, an n-InP cladlayer 7 a (the thickness of 0.2 μm, the doping concentration of 1×10¹⁸cm⁻³), an n-type InGaAsP-SCH layer 8 a (the PL wavelength of 1.05 μmcomposition, the thickness of 50 nm+the PL wavelength of 1.13 μmcomposition, the thickness of 10 nm), a MQW layer 9 (the number ofquantum well of six, the PL wavelength of about 1.3 μm) consisting of anInGaAsP quantum well structure layer (the thickness of 5 nm) with +0.7%compressive strain and an InGaAsP barrier layer (the PL wavelength of1.13 μm composition, the thickness of 8 nm), a p-type InGa AsP-SCH layer8 b (the PL wavelength of 1.13 μm composition, the thickness of 10nm+the PL wavelength of 1.05 μm composition, the thickness of 50 nm))and a p-InP clad layer 7 b (the thickness of 0.1 μm, the dopingconcentration of 7×10¹⁷ cm⁻³) are successively deposited under thecomposition and the layer thickness design value in the rear gainwaveguide portion 17 (WM=50.0 μm, Wo=2.0 μm), as illustrated in FIG.13B.

Subsequently, after SiO₂ mask 2 a is removed, another SiO₂ mask 10 isformed only on the mesa type optical waveguide including the activelayer by the use of the CVD method and the photo-lithography method, asillustrated in FIG. 13C. Thereafter, a current blocking structureconsisting of a p-InP blocking layer 11 a (the thickness of 0.5 μm, thedoping concentration of 3×10¹⁷ cm ⁻³) and an n-InP blocking layer 11 b(the thickness of 0.7 μm, the doping concentration of 1×10¹⁸ cm⁻³) issuccessively deposited.

Further, after the SiO₂ mask 10 is removed, a p-InP embedding layer 12and a p-InGaAs contact layer 13 a are successively deposited, asillustrated in FIG. 13D. Finally, electrodes 14 are formed on upper andlower surfaces of the substrate 1 a, and is cleaved to the device lengthof 450 μm. Thus, the semiconductor optical amplifier having the facet ofthe window structure is finally obtained, as illustrated in FIG. 13D.

(Fourth Example)

Referring to FIGS. 14A through 14D, description will be made about asemiconductor optical device according to a fourth example of thisinvention. Herein, the fourth example corresponds to the thirdembodiment illustrated in FIG. 5.

As shown in FIG. 14A, a SiO₂ film having the layer thickness of 0.05 μmis deposited by the use of the CVD method on an n-InP substrate 1 a andis patterned by the use of the photo-lithography method and thedry-etching method to form a SiO₂ mask 2 a. The mask width Wm and theopening width Wo of the mask 2 a are set to 50.0 μm and 2.0 μm in theDFB laser portion 20 having the length of 180 μm, respectively.

Further, the mask width WM and the opening width Wo gradually becomesmaller in the taper portion 5 of the length of 150 μm from Wm=35.0 μmand Wo=2.0 μm in the electrical absorb type modulator portion 19 havingthe length of 160 μm, and are set to 1.5 μm and 1.0 μm in the forwardstraight portion 6 having the length 10 μm, respectively.

On the n-InP substrate 1 a having such a SiO₂ mask 2 a, an n-InP cladlayer 7 a (the thickness of 0.2 μm, the doping concentration of 1×10¹⁸cm⁻³), an n-type InGaAsP-SCH layer 8 a (the PL wavelength of 1.13 μmcomposition, the thickness of 50 nm), a MQW layer 9 (the number ofquantum well of seven, the PL wavelength of about 1.3 μm) consisting ofan InGaAsP quantum well structure layer (the thickness of 5 nm) with+1.0% compressive strain and an InGaAsP barrier layer (the PL wavelengthof 1.1 μm composition, the thickness of 8 nm), a p-type InGaAsP-SCHlayer 8 b (the PL wavelength of 1.13 μm composition, the thickness of 50nm) and a p-InP clad layer 7 b (the thickness of 0.1 μm, the dopingconcentration of 7×10¹⁷ cm⁻³) are successively deposited under thecomposition and the layer thickness design value in the DFB laserportion 20 (WM=50.0 μm, Wo=2.0 μm), as illustrated in FIG. 14B. Herein,the layer thickness and band gap wavelength of the active layer which iscombined with the electrical absorb type modulator portion 19, the SCHlayers 8 a, 8 b and the MQW layer 9 in the taper portion 5 are equal to160 nm and 1.27 μm, respectively.

Subsequently, after a diffraction grating (not shown ) is formed on thep-InP clad layer 7 b of the DFB laser portion 20, another SiO₂ mask 10for selectively growing a buried layer having the opening width of 1 μmis formed on the n-InP substrate 1 a by the use of the CVD method, thephoto-lithography method and the dry-etching method, as illustrated inFIG. 14C.

Next, a p-InP buried layer 12 and a p-InGaAs contact layer 13 a aresuccessively deposited on the substrate 1 a, as illustrated in FIG. 14C.

Successively, an insulating film 15 is deposited by the use of the CVDmethod, as shown in FIG. 14D. After an opening is formed on the p-InGaAscontact layer 13 in the electrical absorb type modulator portion 19 andthe DFB laser portion 20, electrodes 14 are formed on upper and lowersurfaces of the substrate 1 a. Thus, the modulator integration DFB laseris completed in this example, as illustrated in FIG. 14.

Although the preferred embodiments has been described as mentionedbefore, this invention is not limited to above embodiments.

For instance, although the Fabry-perot type resonator and the DFB laserare exemplified as the laser, the DBR laser may be used. Further, theoscillation wavelength is equal to 1.3 μm in the above embodiments.Alternatively, any wavelength band including, for example, a visiblewavelength band, such as 1.55 μm, 1.65 μm, 0.98 μm and 0.68 μm, may beused.

Moreover, although the MQW structure having the compressive strainedquantum well layer is used in the above embodiments, the MQW structurehaving an distortion, a distortion compensation type MQW structure and abulk active layer may be used. Further, AlGaInAs/InP system, AlGaAs/GaAssystem, AlGaInP/GaInP system may be used as the materials other than theInGaAsP/InP system in the above-mentioned embodiments.

Further, a buried structure having, for example, a semi-insulating Fedoped InP may be used instead of the pnpn type current block structureof the homo-embedding structure of the P-InP. Moreover, the conductivitytype of the substrate is not limited to the n-type in the aboveemobdiment, and may be a p-type.

What is claimed is:
 1. A semiconductor optical device comprising: a gainregion for oscillating a laser light beam; a spot-size conversion regionfor converting a spot-size of the laser light beam emitted from saidgain region; and an optical waveguide along said gain region and saidspot-size conversion region, wherein said optical waveguide has awaveguide taper portion which is formed by the use of a selective growthmask in said spot-size conversion region and has a width and a facet sothat the width gradually becomes narrower towards the facet and, as aresult, the waveguide taper portion is tapered along a direction fromsaid gain region towards the facet.
 2. A device a claimed in claim 1,wherein: said optical waveguide further has a rear straight portion at arear portion of said device to serve as said gain region.
 3. A device asclaimed in claim 1, wherein: said optical waveguide has a forwardstraight portion at a forward facet of said device.
 4. A device asclaimed in claim 1, wherein: said optical waveguide further has anelectrical absorb type modulator portion coupled to a rear end of saidwaveguide taper portion, and a distributed feedback laser portioncoupled to a rear end of said electrical absorb type modulator portion,said distributed feedback laser portion being wider in width than saidelectrical absorb type modulator portion.
 5. A device as claimed inclaim 1, wherein: said optical waveguide has a layer thickness and aband gap wavelength in said waveguide taper portion, said layerthickness and the band gap wavelength being kept substantially constant,said waveguide taper portion serving as an active waveguide.
 6. A deviceas claimed in claim 1, wherein: said optical waveguide has a layerthickness and a band gap wavelength in said waveguide taper portion, thelayer thickness becomes thinner to shorten the band gap wavelength asthe width of the optical waveguide becomes narrower towards the facet,said waveguide taper portion serving as a passive waveguide.
 7. A deviceas claimed in claim 1, wherein: said selective growth mask has a masktaper portion which has a mask width and a mask opening width; both themask width and the mask opening width gradually becoming narrower insaid mask taper portion towards the facet.
 8. A device as claim dinclaim 4, wherein: said selective growth mask has a first straightportion which corresponds to said electrical absorb type modulatorportion and a second straight portion which corresponds to saiddistributed feedback laser portion, the second straight portion having awider width and a higher layer thickness than the first straightportion.
 9. A device as claimed in claim 1, wherein: said opticalwaveguide is composed of clad layers and an active layer having aquantum well structure therebetween.
 10. A semiconductor optical devicecomprising: a gain region for oscillating a laser light beam; aspot-size conversion region for converting a spot-size of the laserlight beam emitted from said gain region; and an optical waveguide alongsaid gain region and said spot-sized conversion region, wherein saidoptical waveguide comprises a first waveguide taper portion which isformed by the use of selective growth mask in said spot-size conversionregion at a front facet of said device and which has a first width sothat the first width gradually becomes narrower towards the front facetand, as a result, the first waveguide taper portion is tapered along adirection from said gain region towards the front facet; and a secondwaveguide taper portion which is formed by the use of the selectivegrowth mask in said spot-size conversion region at a rear facet of saiddevice and which has a second width so that the second width graduallybecomes narrower towards the rear facet and, as a result, the secondwaveguide taper portion is tapered along a direction from said gainregion towards the rear facet.
 11. A device as claimed in claim 10,wherein: said optical waveguide further comprises a middle straightportion between said first and second waveguide taper portions to serveas said gain region.
 12. A device as claimed in claim 10, wherein: saidoptical waveguide has a front straight portion at the front facet and arear straight portion at the rear facet.
 13. A device as claimed inclaim 10, wherein: said optical waveguide has further window structureportions at the front facet and the rear facet.
 14. A device as claimedin claim 10, wherein: said optical waveguide has a first layer thicknessand a first band gap wavelength in said first waveguide taper portion,and said optical waveguide has a second layer thickness and a secondband gap wavelength in said second waveguide taper portion, the firstlayer thickness and the first band gap wavelength being keptsubstantially constant, the second layer thickness and the second bandgap wavelength being kept substantially constant, said optical waveguideserving as an active waveguide.
 15. A device as claimed in claim 10,wherein: said first optical waveguide has a first layer thickness and afirst band gap wavelength in said first waveguide taper portion, andsaid second optical waveguide has a second layer thickness and a secondband gap wavelength in said second waveguide taper portion, the firstlayer thickness becoming thinner to shorten the first band gapwavelength as the width of the optical waveguide becoming narrowertowards the front facet, the second layer thickness becoming thinner toshorten the first band gap wavelength as the width of the opticalwaveguide becoming narrower towards the rear facet, said opticalwaveguide serving as a passive waveguide.
 16. A device as claimed inclaim 10, wherein: said selective growth mask comprises: a first masktaper portion which has a first mask width and a first mask openingwidth; a second mask taper portion which has a second mask width and asecond mask opening width; both the first mask width and the first maskopening width gradually becoming narrower in said first mask taperportion towards the front facet, both the second mask width and thesecond mask opening width gradually becoming narrower in said secondmask tape portion towards the rear facet.
 17. A device as claimed incliam 10, wherein: said optical waveguide is composed of clad layers andan active layer having a quantum well structure therebetween.